The present invention relates to a method of forming a semiconductor device, and more particularly to a method of forming an aluminum interconnection layer extending not only over an insulating layer or an inter-layer insulator but also within a contact hole, a through hole or a via hole formed in the insulating layer or the inter-layer insulator.
In semiconductor devices such as intenerated circuits (ICs) and large scale integrated circuits (LSIs), contact holes and through holes are formed in an insulating layer and an inter-layer insulator for filling a conductive material such as aluminum within the contact holes and the through holes and subsequent forming of an interconnection layer which extends over the insulating layer and the inter-layer insulator so that the interconnection layer is in contact with the conductive material such as aluminum within the contact holes and the through holes.
As the requirements for further increase in high integration of the integrated circuits and for further improvement in high speed performances thereof have been on the increase, the required design rule is scaled down, whereby diameters of the contact holes and the through holes are also required to be reduced. This reduction in diameters of the contact holes and the through holes causes further increase in aspect ratio of the contact holes and the through holes. An aspect ratio is defined to be a ratio of a thickness of the inter-layer insulator to the diameter, wherein the contact holes or the through holes are formed in the inter-layer insulator.
Typical two available methods for filling an aluminum into the contact holes or the through holes have been known in the art. A first method is a chemical vapor deposition method. A second method is a physical vapor deposition method such as a sputtering method.
The chemical vapor deposition method can obtain such a good step coverage as to permit an aluminum layer to be deposited not only to extend over the inter-layer insulator but also to fill the contact holes or the through holes. The aluminum layer deposited by the chemical vapor deposition method is likely to be deteriorated in morphology so that a void is formed in the aluminum layer filled with the contact holes or the through holes.
FIG. 1 is a fragmentary cross sectional elevation view illustrative of an aluminum layer formed by the conventional chemical vapor deposition, wherein the aluminum layer not only extends over an inter-layer insulator but also is filled within a contact hole and a void is formed in the aluminum layer within the contact hole. An inter-layer insulator 62 is formed on the silicon substrate 61. A contact hole 63 is formed in the inter-layer insulator 62 so as a part of the surface of the silicon substrate 61 is shown through the contact hole 63. An aluminum layer 64 is deposited by a chemical vapor deposition method so that the aluminum layer 64 not only extends over the inter-layer insulator 62 but also is filled within the contact hole 63, wherein a void 65 is formed in the aluminum layer 64 within the contact hole 63.
The inter-layer insulator is often formed which extends over a step-shaped surface which has been formed by a patterning of an lower level interconnection layer on an insulating layer. If the contact hole is formed in the inter-layer insulator so that the contact hole is displaced and positioned over the step, then the aluminum layer formed within the displaced contact hole may have a void, whereby an aluminum contact layer within the displaced contact hole may no longer have a reliability in connection.
In order to solve the above problems, it was proposed to carry out the chemical vapor deposition processes of aluminum two times, wherein following each chemical vapor deposition process, a high pressure reflow process is carried out to collapse the voids. This second conventional method is disclosed in Japanese laid-open patent publication No. 8-293552. FIGS. 2A and 2B are fragmentary cross sectional elevation views illustrative of a second conventional method wherein chemical vapor deposition processes of aluminum are carried out two times to form an aluminum interconnection layer out only extending over an inter-layer insulator but also fills within a contact hole which is, however, displaced so that the contact hole is positioned over a step formed by an edge of a bottom level interconnection over a substrate.
With reference to FIG. 2A, a bottom level interconnection 71 is formed on a substrate. An inter-layer insulator 72 is formed which extends over the bottom level interconnection 71 and the substrate so that the bottom level interconnection 71 is completely buried within the inter-layer insulator 72. A contact hole 73 is formed in the inter-layer insulator 72. However, the contact hole 73 is displaced so that the contact hole 73 is positioned over a step formed by an edge of the bottom level interconnection 71. A first aluminum layer 74 is deposited by a first chemical vapor deposition process. A first high pressure reflow process is subsequently carried out at a temperature of 400.degree. C. under a pressure of not less than 600 MPa, whereby the first aluminum layer 74 is pushed down to a deep portion of the contact hole having a high aspect ratio.
With reference to FIG. 2B, a second aluminum layer 75 is deposited by a second chemical vapor deposition process. A second high pressure reflow process is subsequently carried out thereby the contact hole having the high aspect ratio is completely filled by the laminations of the first and second aluminum layers 74 and 75 without forming any void. As a result, a void free aluminum contact is formed within the contact hole having the high aspect ratio.
FIGS. 3A and 3B are fragmentary cross sectional elevation views illustrative of a first conventional sputtering method for forming an aluminum interconnection layer not only extending over an inter-layer insulator but also fills within a through hole formed in the inter-layer insulator.
With reference to FIG. 3A, an inter-layer insulator 82 is formed on a silicon substrate 81. A through hole 83 having a high aspect ratio is formed in the inter-layer insulator 82. An aluminum layer 84 is deposited by a sputtering method which extends not only over the inter-layer insulator 82 but also in the through hole 83. However, the sputtering method is inferior in step coverage, for which reason the aluminum layer 84 does not fill the through hole 83.
With reference to FIG. 3B, a reflow process is carried out to the deposited aluminum layer 84 so that the deposited aluminum layer 84 fills the through hole 83 whilst a depression is formed on the re-flowed aluminum layer over the through hole 83, because a moving distance of aluminum atoms is small when the sputtering method is used whereby only aluminum atoms in the vicinity of the through holes may move into the through hole 83. For the above reasons, the sputtering method is applicable only to the through hole of a low aspect ratio of not higher than 3. Further, the above sputtering method is inapplicable to when the inter-layer insulator is made of a dielectric having a low thermal stability. Generally, the dielectric having a high dielectric constant has a high dielectric constant and the dielectric having a low dielectric contact has a low dielectric constant. Thus, the sputtering method is applicable to only the dielectric having a high dielectric constant. If, however, the dielectric having a high dielectric constant is used for the inter-layer insulator, then a problem with a large parasitic capacitance between interconnections separated by the inter-layer insulator may be raised. This large parasitic capacitance between interconnections separated by the inter-layer insulator causes a delay in transmission of signals on the interconnections.
In Japanese laid-open patent publication No. 7-130851, it is disclosed to solve the above problems. FIGS. 4A through 4D are fragmentary cross sectional elevation views illustrative of a second conventional sputtering method for forming an aluminum interconnection not only extending over an inter-layer insulator but also fills a contact hole formed in the inter-layer insulator.
With reference to FIG. 4A, a base interconnection 91 is formed on a silicon substrate. An inter-layer insulator 93 is formed over the base interconnection 91 and over the silicon substrate so that the base interconnection 91 is completely buried within the inter-layer insulator 93. A contact hole 92 is formed in the inter-layer insulator 93 so that the contact hole 92 is positioned over the base interconnection 91 whereby a part of the base interconnection 91 is shown through the contact hole 92. A base layer 94 is deposited which extends over the top surface of the inter-layer insulator 93 and also extends on vertical walls of the contact hole 92 and the bottom of the contact hole 92. The base layer 92 has a property for facilitation of application of an aluminum-based material. Subsequently, a sputtering process is carried out to deposit an aluminum-based thin layer 95 on the base layer 94 except on the vertical wall of the contact hole 92.
With reference to FIG. 4B, a heat treatment is carried out at a temperature of 500.degree. C. so that the aluminum-based thin layer 95 shows a reaction with the base layer 94 so that the aluminum-based thin layer 95 is re-flowed through the vertical walls toward the bottom of the contact hole 92.
With reference to FIG. 4C, after the heat treatment has been continued for 30 seconds, the re-flowing aluminum-based thin layer 95 reaches the bottom and deep portion of the contact hole 92.
With reference to FIG. 4D, a aluminum-based layer 96 is further deposited on the aluminum-based thin layer 95 by a sputtering method at a high temperature so that the contact hole 92 is completely filled by laminations of the aluminum-based thin layer 95 and the aluminum-based layer 96.
The above conventional methods of the chemical vapor deposition in combination with the high pressure re-flow process have the flowing disadvantages. A chemical vapor deposition is carried out under a vacuum whilst the high pressure re-flow process is carried out under a high pressure which is higher than the vacuum of the chemical vapor deposition by 1.times.10.sup.7 times. A chemical vapor deposition system and a high pressure re-flow system are connected under an inert gas atmosphere or a vacuum. This connection mechanism enlarges the system. Further, it takes a long time to carry a wafer between the systems. Those result in a large increase in manufacturing cost of the semiconductor device.
In the meantime, the above other conventional method of the physical vapor deposition such as the sputtering method is combination with the re-flow process also has the above disadvantages as engaged with the conventional methods of the chemical vapor deposition. A sputtering process is carried out under a vacuum whilst the high pressure re-flow process is carried out under a high pressure which is largely different from each other. A sputtering system and a high pressure re-flow system are connected under an inert gas atmosphere or a vacuum. This connection mechanism enlarges the system. Further, it takes a long time to carry a wafer between the both systems. Those result in a large increase in manufacturing cost of the semiconductor device. The above other conventional method of the physical vapor deposition such as the sputtering method in combination with the re-flow process further has the following disadvantages. As a result of the reaction of the aluminum layer with the base layer, a highly resistive alloy layer is formed, for which reason a resistance of the interconnection is increased, whereby a high speed performance of the semiconductor device is deteriorated.
In the above circumstances, it had been required to develop a novel method of forming an aluminum interconnection layer extending not only over an inter-layer insulator but also within a contact hole, or a through hole or a via hole free from the above disadvantages.